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The study of reaction mechanisms in materials for Li-ion batteries mainly involves localization of lighter elements like Li, O, or even H in the structure. Thus, in order to facilitate in situ localization of lighter elements and in situ study of structural evolution in the electrode materials, a circular in situ neutron diffraction cell capable of cycling small amounts of electrode materials (0.2-0.3 g) was developed for primary use at the D20 beamline at ILL, Grenoble, France. The circular cell design was tested using LiFePO 4 and graphite as the model electrode materials. The effect of using deuterated electrolyte versus protonated electrolyte on the quality of the in situ neutron diffraction data was also investigated. First in situ neutron powder diffraction measurements at ILL, Grenoble, were successfully conducted where each neutron diffraction pattern was recorded in only 24 min, delivering very good time resolution. It was also found that a circular cell design holding only a small amount of material soaked in deuterated electrolyte was best to perform quantitative analysis using the Rietveld method over the complete 2 theta range. The pattern shows no apparent anisotropic absorption of the diffracted neutron beams.

The effect of Mg-substitution in Li(Ni,Co,Al)O 2 during overcharge was studied by scanning electron microscopy (SEM) observations and in situ synchrotron X-ray Diffraction (XRD) measurements. The higher Mg-substituted samples showed higher electrochemical stability during the overcharge cycling. After overcharge, the non Mg-substituted sample featured many cracks, not only at the grain boundaries in secondary particles but also inside the primary particles. This morphological change is assigned to the drastic shrinking of the c-axis during overcharge. The Mg-substitution suppressed the c-axis shrinking and consequently avoided the cracking of the particles. This effect of Mg-substitution in Li(Ni,Co,Al)O 2 correlates with the electrochemical stability during the overcharge cycling.The durability of lithium-ion batteries when exposed to overcharge is a major concern of battery developers and users. Not to mention, it is a dream to develop a lithium-ion battery which doesn't require any protection circuit or system. Over the past decades, a considerable number of studies have been conducted on this issue by thermometric and/or gasometric analyses. 1-6 Most researchers seem to agree that the stability of overcharged positive electrode materials is one of the key factors 3-6 for the safety of lithium-ion batteries.LiNiO 2 -based materials such as Li(Ni,Co,Al)O 2 have been widely studied because they have a large charge capacity when used as a positive electrode material. 7-10 However, the processes in batteries utilizing Li(Ni,Co,Al)O 2 during abuse conditions, such as overcharge cycling up to 5.0 V, are not clear in detail. Our previous research has shown that a small amount of Mg substitution in Li(Ni,Co,Al)O 2 has much impact on the electrochemical behavior. 11 To be concrete, the capacity retention during cycling at 60 • C improved and the increase in battery resistance was reduced by suppressing the increase in charge-transfer resistance of the positive electrode. Other papers have also reported the positive effect of the Mg substitution in a similar compound, LiNi 0.8 Co 0.2 O 2 , 12 however, detailed information about overcharge behavior is still lacking.Our objectives in this study are to investigate overcharge behavior of Mg-substituted Li(Ni,Co,Al)O 2 and to clarify the effect of Mg substitution on the same. We also aim to clarify the influence of this Mg substitution on the subsequent overcharge cycling. To explore the behavior under such abusive conditions, analyses "after" overcharge would be not enough; analyses "during" overcharge are necessary because the active materials are obviously unstable and in a non-equilibrium state at high potentials. From these viewpoints, in situ synchrotron XRD is the method of choice to investigate the overcharge behavior of positive electrode materials. 13, 14 Additionally, we have previously reported that some cracks were observed at grain boundaries in secondary particles of LiNi 0.80 Co 0.15 Al 0.05 O 2 after standard cycles. 9 This morphological behavior deserves ca...

Structural investigation was performed on a range of NMC materials, Li 1þx (Ni 1/3 Mn 1/3 Co 1/3 ) 1-x O 2 (0 x 0.13), using in situ X-ray synchrotron diffraction while electrochemically cycling them between 2.5 and 5.0 V vs. Li/Li þ . From the analysis of the obtained data, an irreversible phase transformation of part of the starting O3 phase (R 3 m) to the O1 phase (P 3 m1) was observed for all the NMC samples, where the O1 phase was detected at more negative potentials with increasing overlithiation content. Moreover a second irreversible reaction was detected at positive potentials using ex situ X-ray synchrotron diffraction for overlithiated NMC (x ¼ 0.1) that involved loss of Li 2 MnO 3 ordering in the structure.

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